They're all around us — the living dead, or mostly dead. Just waiting to wreak havoc.

How's that for a pre-Halloween setup?

But it's true. "Zombie trees," as arborists call them, are all over the place. Unusual weather patterns cause added stress for aging trees that can topple and result in serious damage and injury.

In a city like Memphis, filled with towering oaks, it's reason to be alert.

"They've been around 85, 90 or maybe 100 years," local arborist Nathan Baker said of the city's treeline. "They're 'overly mature,' meaning they've become really big and don't have the vitality anymore of younger trees."

Throw in drought conditions or high winds, and it can spell big trouble — or at least big disruption — if and when the trees can't take it any longer. Trees falling on power lines frequently cause outages, and multiple people have been killed or injured or sustained severe property damage as the result of falling trees during storms.

'They are dead but just don't know it yet'

So to back up, what exactly is a "zombie tree"?

"Some are in the process of dying, and some may actually be dead trees already," said Baker, of Jones Brothers Tree and Landscape. "That's really what we're talking about when we say 'zombie trees.'"

Paul Johnson, the urban and community forestry program leader for the Texas A&M Forest Service, put it a bit more dramatically this summer when describing trees that were beginning to succumb 10 months after Hurricane Harvey hit the Houston area hard:

His comments came on the heels of a 30-day period in which at least three people died nationwide from falling trees, including a teen attending a Boy Scout summer camp in Georgia and two journalists in South Carolina who were covering storms.

A close call at Levitt Shell

Locally, a tree fell Sunday night near the Levitt Shell in Overton Park. It narrowly missed guests attending a concert, crushing through a tent and landing on a deck. No one was injured.

It's unclear if the tree that fell, a black cherry, qualified as a "zombie tree," though a Shell worker cleaning up debris Monday said the aging tree had been marked for removal prior to falling.

A tree fell narrowly missing guests at the Levitt Shell in Memphis tonight. As the concert ended, and as people left the tree fell on the deck crushing through the tent. No one was injured. The tree was marked for removal. #MEMPHISpic.twitter.com/KvbMcFPU6U

Explained Baker: "Once a part of a tree dies, water stops moving through the tree and the tree will become brittle. So the wood will dry out and become brittle, and you're at the point where you could start to see some failures."

Age more important than variety

Baker said there's no particular type of tree that's more susceptible. It's more about age, he said, than variety.

Anyone who suspects a tree might be dying or nearly dead and on the verge of causing problems should contact an arborist as a first step, he suggested.

"It's typically a no-cost kind of thing," he said. "A lot of times, the recommendation is just to 'call me next year.' Or they can do some basic tree fertilization to help it endure stressful situations for awhile longer."

Baker said in many cases, it comes down to a personal decision about risk tolerance.

"You've really got to consider what your risk acceptance level is," he said. "If there are just a couple of dead limbs that aren't really close to anything, it's not that big a deal. But if you get to where it's within striking distance of the house or your high-usage areas, then you've got a problem."

How to spot a zombie tree

Arborists say there are some telltale signs to help alert you if zombies are lurking about. Among the things to look for:

Leaf loss from the ends of branches and the top of the tree.

Discoloration of leaves, usually yellow.

Leaves that begin falling prematurely.

If problems are suspected, contact an arborist for an evaluation. Sometimes, it isn't necessary to remove the tree completely — trimming dead branches and fertilizing might help a tree that isn't completely dead survive stress.